Another one ispired by Mauricio Antón's book Sabertooth.
Thylacosmilus atrox was the most famous of the marsupial sabertooths. These weird predators lived in South America during the Miocene and Pliocene, when it was isolated from other continents and evolved a really weird and unique fauna.
The sabers of Thylacosmilus were so big they went all the way up the snout inside it's skull. The roots of the fangs are actually above the creature's eyes. It seems they grew continuously much like the incisors of rodents, which might work as a sort of insurance against breaking a tooth. Also, the bite of this 100 kg animal was apparently weaker than that of a domestic cat.